Hillary Clinton stands poised to claim the Democratic mantle with Tuesday’s primaries including California, overwhelming her rival Bernie Sanders and setting up a historic US presidential election showdown with Republican Donald Trump .

The frontrunner is all but assured of locking in her party’s nomination, a monumental step towards fulfilling a dream of returning to the White House as commander in chief, 16 years after serving as first lady to president Bill Clinton.

The former secretary of state neared the brink of victory with a strong win Sunday in the island territory of Puerto Rico, one day after the US Virgin Islands voted overwhelmingly for her.

But a loss in California, the largest state in the nation, would blunt that momentum and potentially complicate Clinton’s efforts to unify the party.

Clinching the nomination will launch an unprecedented battle between a candidate seeking to break the glass ceiling and become the country’s first female commander in chief, and a provocative billionaire businessman and political neophyte who has changed the face of American campaigning.

Clinton, who like Sanders has stumped relentlessly in the Golden State in recent weeks, signalled the race has run its course and she will prevail as Democratic standard bearer.

“I believe on Tuesday I will have decisively won the popular vote and I will have decisively won the pledged delegate majority,” she told CNN from California.

“After Tuesday I’m going to do everything I can to reach out to try to unify the Democratic Party and I expect Senator Sanders to do the same.”

But Sanders contends Clinton’s lead is largely based on support from so-called super-delegates, party big-wigs who are not bound to vote for a specific candidate, and he proclaimed he will take his nomination fight all the way to the Democratic convention in July.

Sanders, a self-proclaimed democratic socialist, argued Sunday he could persuade many of the more than 500 super-delegates who back Clinton to reverse course and vote for him, adding that it would be “not quite accurate” for Clinton to claim victory by counting super-delegates before the convention.

Sanders has signalled he will use the time between the final Democratic primary — June 14 in the US capital Washington — and the July 25-28 convention to win over super-delegates. A strong Sanders night Tuesday, though, will not deny the inevitable mathematics of Clinton crossing the threshold.She now stands at 2,354 total delegates, according to CNN’s tally, just 29 shy of the number needed for victory.

More than 600 pledged delegates are at stake Tuesday, and Clinton is on a glide path to reach the magic number when results come in from New Jersey, where polling stations close earliest.

She has scheduled a primary night event in Brooklyn, New York.

On Sunday in California she addressed the magnitude of what she is trying to achieve.”I know we’ve never done this before. We’ve never had a woman president,” she told supporters.

At a restaurant in the Los Angeles neighborhood of Inglewood, where Bill Clinton was campaigning for his wife, patron Martin Jones, 73, said he was “100 per cent” behind Hillary.

“She would be coming from a laboratory of experience,” Jones said. “It’s about time we have a woman in charge of our government instead of our kitchen.”

Clinton has recently focused like a laser on Trump.

She unleashed blistering attacks on the Republican in recent days, including a well-received foreign policy speech in which she declared the real estate tycoon’s ideas “dangerously incoherent” and warned he was “temperamentally unfit” to lead the world’s more powerful nation.

Trump blasted back on social media with similar language Sunday.

“Hillary Clinton is unfit to be president. She has bad judgement, poor leadership skills and a very bad and destructive track record. Change!” he posted on Twitter.

It has been a week of mis-steps for the Republican, however. He has assailed the Hispanic-American judge presiding over the fraud case against Trump University, saying his Mexican heritage made him biased against Trump.

His comments have earned rebukes from Republican stalwarts including House Speaker Paul Ryan.

But he doubled down on the concept Sunday.

Asked whether he would feel that a Muslim judge would not be able to treat him fairly given his call for a temporary ban on Muslims entering the United States, Trump told CBS News: “Yeah. That would be possible, absolutely.”